Rapid changes in the precision requirements of various mechanical devices, machines and apparatus have brought to the forefront the need for better control of surface texture of manufactured parts. In combination, advancements in the metal working arts have made the production of higher quality surfaces a possibility and affordable to the metal parts manufacturer. Automotive products are currently designed to rigorous specifications that require proper quality control of critical load bearing surfaces for adequate operation and minimal maintenance.
Microfinishing or superfinishing, as it is known in the art, is a surface finishing process wherein a grinding means is brought to bear against a workpiece which has been previously rough ground. Microfinishing is a low velocity abrading process which generally follows rough grinding and finish grinding. Because microfinishing incorporates lower cutting speeds than grinding, heat and pressure variants may be minimized to provide improved size and geometry control. Those skilled in the art recognize that surface quality or roughness is measured in roughness average values (R.sub.a) wherein R.sub.a is the arithmetical average deviation of minute surface irregularities from hypothetical perfect surfaces. Microfinishing can provide surface quality of approximately 0.025 to 0.25 .mu.m. Load bearing surfaces of crankshafts, cam shafts, power transmission shafts in similar machine components that rotate on journal bearing surfaces generally require this surface finish for satisfactory operation.
Microfinishing may be accomplished using many different known forms or methods. Stone microfinishing uses a stationary honing stone which is brought against a desired bearing surface to be finished. Another method is described as conventional abrasive coated tape microfinishing. In this method a coated abrasive tape is brought into contact with a rotating bearing surface. As the part is rotated, the abrasive material reduces roughness of the surface. In the conventional process, the tape is brought into contact with the rotating surface by pressure exerted by compressible elastomeric inserts, typically made from urethane plastic materials. In yet a another method of microfinishing, a rigid insert is used to press abrasive coated tape or cloth material into contact with the rotating bearing surface.
Burnishing is the smoothing of the surface finish of workpieces by frictional, direct contact of a fixed or rotating tool under pressure. Burnishing is used as a finishing operation after a workpiece has been machined or ground to eliminate minute surface irregularities. Burnishing is to be distinguished from spinning operations wherein the tool is urged against a workpiece blank which is deformed into a desired shape usually in a series of passes with substantial deformation of material. A true roller burnishing operation is a cold working operation which does not remove material but compresses material. Roller burnishing compresses the microscopic peaks and valleys that are inherently left behind on the workpiece in metal turning operations.
Roller burnishing may provide surface quality of approximately 0.40 to 0.20 .mu.m. Roller burnishing also increases surface hardness characteristics and improves overall surface geometry. Roller burnishing is capable of improving overall surface hardness by 10 to 30 percent over the initial hardness characteristics.
It is known that in the burnishing operation, microscopic peaks are broken off or deformed down into the adjacent microscopic valleys of the surface of the metal workpiece. It is also known that after repeated loads are placed on bearing surfaces, these previously burnished surfaces including the compacted or deformed microscopic peaks may fracture or become dislodged from the surface of the workpiece. These unattached microscopic contaminants are known to cause wear and breakdown of the bearing surface of the workpiece and mating parts over time.
It is also known that microfinishing, when applied properly and at the correct pressure and time variances, will remove the microscopic peaks from the surface to create the sought after microfinished surface. These peaks are then transported off the surface by the particular abrasive method used, such as the abrasive tape indexing method or changes to the honing stone materials or subsequent cleansing of the workpiece prior to installation.
The present invention solves the above noted problems and others in a manner not disclosed in the prior art.